java.lang.IllegalStateException: Listener 136505312 is not attached to J2SEProjectPlatform 732243788 for Project JavaApplication1.
at org.netbeans.modules.javawebstart.JWSProjectOpenHook$PlatformListener.detach(JWSProjectOpenHook.java:219)
at org.netbeans.modules.javawebstart.JWSProjectOpenHook.projectClosed(JWSProjectOpenHook.java:164)
at org.netbeans.spi.project.ui.ProjectOpenedHook$1.projectClosed(ProjectOpenedHook.java:89)
at org.netbeans.spi.project.ui.support.UILookupMergerSupport$OpenHookImpl.projectClosed(UILookupMergerSupport.java:260)
at org.netbeans.spi.project.ui.ProjectOpenedHook$1.projectClosed(ProjectOpenedHook.java:89)
[catch] at org.netbeans.modules.project.ui.OpenProjectList.notifyClosed(OpenProjectList.java:1253)
at org.netbeans.modules.project.ui.OpenProjectList.access$2500(OpenProjectList.java:137)
at org.netbeans.modules.project.ui.OpenProjectList$10.run(OpenProjectList.java:922)
at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1432)
at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2042)

The exception is caused by en existence of multiple instances of the JWSProjectOpenHook.
The projectOpened is called on different instance than the projectClosed which causes the ISE.
The problem is that the lookup result of a lookup created by Lookups.forPath() for a short while has no items (neither old ones nor new ones) in this time the DelegatingLookupImpl deletes the old map and in next run it recreates the object instances by calling createAdditionalLookup on LookupProviders.
The issue is a race, so it's not 100% reproduceable but it happens quite common with following steps:
1) Open IDE with ergonomics and empty user dir
2) Create a new J2SE Project (activates J2SE cluster)
3) Create a new Web Proejct (activates Web cluster)
4) Close the J2SE Project -> ISE is thrown

The issue can be "fixed" by an attached patch which caches the created additional Lookups.
However this is rather workaround and it would be better to fix the Lookups.forPath Lookup as the patch is sensitive to possible Lookup.equals() and similar problem may exist on different places.

The call to the POH is asymmetric -> several instances were created by the Lookup. It can be caused by Project's merged lookup, however previously the reason was this issue. Unfortunately I cannot find the messages.log in the exception report of the issue #234463 to prove it.